We previously developed an experimental "rescue" system for respiratory syncytial virus (RSV) based on a cDNA-encoded "minigenome" bearing a foreign marker gene, such as that encoding chloramphenicol acetyl transferase (CAT). These minigenomes can be rendered competent for transcription, replication and incorporation into virions by complementation with proteins supplied by standard RSV helper (accompanying report). We are now attempting to modify this system such that the helper RSV is replaced by RSV proteins synthesized from vectors. This would allow us to supply the proteins in different combinations and amounts. It would then be possible to identify the viral proteins required for the major steps in the viral replicative cycle, in particular transcription, replication and virion formation. This would assign functions to the viral proteins, a number of which are completely uncharacterized. Also, the system could be used to perform detailed structure-function studies of individual proteins involved in all stages of the replicative cycle and would be the method of choice for rescuing complete infectious virus from cDNA-encoded genome RNA.